This invention relates in general to spray guns and, more specifically, to an improved high volume/low pressure warm output spray gun of improved efficiency.
Spray guns in which compressed air is mixed with a liquid, atomizing the liquid and projecting it as very small droplets against a surface to be coated have been in widespread use for very many years. Typically, spray guns are used for painting, application of pesticides and the like.
Due to environmental concerns, greater importance is being given to paint spray systems which use less solvent, reduce overspray and migration and generate fewer emissions and pollutants providing a cleaner, safer environment for workers. In addition, reduced overspray results in cost savings.
Conventional and electrostatic air spray systems typically use air pressure ranging from 30 to 90 psi. As compressed air exits the gun nozzle, it suddenly expands as it returns to atmospheric conditions. While this sudden expansion aids atomization, this "exploding" or "blasting" effect propels the paint droplets at high velocity, causing overspray and bounce-back. Paint "fog" often occurs, making it difficult for the painter to see his work. In masked painting, this high spray velocity may force paint under the maskant, causing rejection or extra re-work. Overspray results in wasted material, contamination of spray booth filters and increased requirements for cleaning spray booths. Typical of these conventional spray guns are those disclosed by Bramsen, et al in U.S. Pat. No. 1,797,209 and Sykes in U.S. Pat. No 2,888,207.
Two different approaches to this problem have recently been developed. One uses a turbine compressor to produce a high volume of low pressure, warm air to the spray gun, while the other uses a venturi to draw in and mix outside air in the region of this spray gun with high pressure compressed air to provide a greater volume of lower pressure air to the spray gun.
Typical of the turbine systems is that described by Muck in U.S. Pat. No. 4,565,488 and available from the Bessam-Aire Company under the Accuspray BE-80 designation. A turbine provides warm air to a spray gun at 7 psi or less. While these sytems are effective in reducing overspray and paint bounce-back, they are relatively expensive since a fixed or cart mounted turbine for each one or two spray guns is required. Also, temperature and pressure of the air going into the spray gun is difficult to adjust and the warm air going through a long air hose may cool before reaching the spray gun.
In another approach, a venturi is used in the air line to the spray gun, either in or near the gun handle. Such a system is described by Farnsteiner in U.S. Pat. No. 3,796,376. High pressure air is directed through the venturi pulling in and mixing a large volume of ambient air while reducing pressure. Venturi systems are available, for example, from the Lex-Aire Co. While these systems reduce paint mist and overspray, several problems remain. They are incapable of warming the air entering the spray gun and will, if anything, cool the air. Also, since ambient air is pulled in from adjacent the spray gun, any paint mist remaining will be pulled into the gun. The filters used to removed paint mist, dust or the like will tend to clog, gradually reducing the quantity of outside air, changing the air-to-paint ratio, making uniform paint application difficult.
Thus, there is a continuing need for improved high volume, low pressure, spraying systems which are lightweight, convenient and economical.